Week 5 seminar Cell Biology: Endomembrane Trafficking and Golgi Apparatus
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Questions and Answers

What role do the carbohydrates attached to lysosomal membrane proteins play?

  • They form a protective layer preventing digestive enzymes from breaking them down. (correct)
  • They function as binding sites for external molecules.
  • They support the structural integrity of lysosomes.
  • They enhance enzyme activity within lysosomes.

    • What can be a consequence of excessive release of lysosomal enzymes by white blood cells?

  • Degradation of joint tissues leading to inflammatory diseases. (correct)
  • Increased protein synthesis in joints.
  • Formation of new synovial fluid.
  • Enhanced immune response against pathogens.

    • Which of the following statements is true about peroxisomes?

  • They are primarily found in prokaryotic cells.
  • They are derived from the endoplasmic reticulum.
  • They are involved in oxidative reactions using molecular oxygen. (correct)
  • They are bounded by double membranes.

    • What enzyme is essential for the degradation of hydrogen peroxide in peroxisomes?

    <p>Catalase.</p> Signup and view all the answers

    How much of fatty acid oxidation in animal tissues occurs in peroxisomes?

    <p>25-50%</p> Signup and view all the answers

    What substance does peroxisomal β oxidation primarily generate?

    <p>Acetyl-CoA.</p> Signup and view all the answers

    In which type of cells are peroxisomes especially prominent?

    <p>Mammalian kidney and liver cells.</p> Signup and view all the answers

    What is a key function of peroxisomes related to oxidative stress?

    <p>Detoxifying reactive oxygen species.</p> Signup and view all the answers

    Which of the following correctly distinguishes peroxisomes from lysosomes?

    <p>Peroxisomes perform oxidative reactions while lysosomes primarily digest macromolecules.</p> Signup and view all the answers

    What is the fate of the primary product of β oxidation in animal cells?

    <p>It enters mitochondrial respiration.</p> Signup and view all the answers

    What role do protein tags play in endomembrane trafficking?

    <p>They direct proteins to specific transport vesicles.</p> Signup and view all the answers

    Which proteins are specifically recognized and targeted in the trans-Golgi for lysosomal delivery?

    <p>Proteins with mannose-6-phosphate tags.</p> Signup and view all the answers

    How do 'early' endosomes differ from 'late' endosomes in function?

    <p>Early endosomes sort materials, while late endosomes fuse with lysosomes.</p> Signup and view all the answers

    What is the primary function of lysosomal enzymes?

    <p>To degrade biological macromolecules.</p> Signup and view all the answers

    Why do lysosomes not digest themselves?

    <p>They have unique protective proteins.</p> Signup and view all the answers

    What mechanism do plant cells use in the Golgi apparatus involving polysaccharides?

    <p>They synthesize polysaccharides for the cell wall.</p> Signup and view all the answers

    What does endocytosis refer to in the context of endosome formation?

    <p>The uptake of materials into the cell.</p> Signup and view all the answers

    How are the contents of vesicles sorted in the Golgi apparatus?

    <p>By tags such as amino acid sequences and chemical markers.</p> Signup and view all the answers

    What happens to soluble lysosomal proteins after binding to receptors in the trans-Golgi?

    <p>They are packaged into clathrin-coated vesicles.</p> Signup and view all the answers

    Which component is NOT involved in the trafficking process in the endomembrane system?

    <p>Ribosomes.</p> Signup and view all the answers

    What type of molecule can serve as tags for membrane lipids?

    <p>One or more phosphate groups.</p> Signup and view all the answers

    How do lysosomes contribute to cellular health?

    <p>By degrading damaged or unneeded macromolecules.</p> Signup and view all the answers

    What is a defining characteristic of the pH in the trans-Golgi compartment?

    <p>It favors binding of soluble lysosomal enzymes.</p> Signup and view all the answers

    What is the primary role of clathrin-coated vesicles in the endomembrane system?

    <p>To transport materials within the cell.</p> Signup and view all the answers

    What is the primary function of the glycocalyx on lysosomal membrane proteins?

    <p>The glycocalyx serves as a protective layer, preventing lysosomal enzymes from degrading the membrane proteins.</p> Signup and view all the answers

    How do lysosomes facilitate extracellular digestion?

    <p>Lysosomes can discharge their enzymes by exocytosis, enabling the digestion of materials outside the cell.</p> Signup and view all the answers

    In what types of cells are peroxisomes particularly abundant?

    <p>Peroxisomes are especially prominent in mammalian kidney and liver cells, as well as in plant cells such as algae and germinating seedlings.</p> Signup and view all the answers

    What toxic compound do peroxisomes help to degrade, and which enzyme facilitates this process?

    <p>Peroxisomes degrade hydrogen peroxide (H2O2) using the enzyme catalase.</p> Signup and view all the answers

    What percentage of fatty acid oxidation occurs in peroxisomes in animal tissues?

    <p>About 25–50% of fatty acid oxidation occurs in peroxisomes.</p> Signup and view all the answers

    How do peroxisomes differ from lysosomes in terms of their origin?

    <p>Peroxisomes are not derived from the endoplasmic reticulum and are not part of the endomembrane system, unlike lysosomes.</p> Signup and view all the answers

    What is the significance of the localized generation and degradation of hydrogen peroxide within peroxisomes?

    <p>This localization prevents toxic exposure of H2O2 to other cellular components, ensuring cellular safety.</p> Signup and view all the answers

    What primary product does β-oxidation produce in peroxisomes, and what happens to this product?

    <p>The primary product of β-oxidation is acetyl-CoA, which is exported to the cytosol for energy production or biosynthetic pathways.</p> Signup and view all the answers

    Why are lysosomes referred to as the cell's 'digestive system'?

    <p>Lysosomes contain digestive enzymes that break down macromolecules and recycle cellular components.</p> Signup and view all the answers

    What challenges might arise if lysosomal enzymes are improperly released into the extracellular space?

    <p>Improper release can lead to tissue damage and contribute to inflammatory diseases, such as rheumatoid arthritis.</p> Signup and view all the answers

    What structural features can serve as tags for proteins in endomembrane trafficking?

    <p>Tags can include a short amino acid sequence, oligosaccharide side chain, or a hydrophobic domain.</p> Signup and view all the answers

    What happens to modified proteins after they are packaged into vesicles from the Golgi apparatus?

    <p>They are sorted and delivered to various cellular locations or secreted from the cell.</p> Signup and view all the answers

    What is the significance of mannose-6-phosphate tags in protein targeting?

    <p>These tags direct soluble lysosomal proteins to lysosomes by binding to specific receptors.</p> Signup and view all the answers

    Explain the role of 'early' endosomes in the endomembrane system.

    <p>'Early' endosomes sort and recycle internalized materials from endocytosis.</p> Signup and view all the answers

    How do lysosomal enzymes contribute to cellular function?

    <p>They digest damaged macromolecules and degrade extracellular materials.</p> Signup and view all the answers

    What distinguishes 'late' endosomes from 'early' endosomes?

    <p>'Late' endosomes fuse with lysosomes to digest materials, while 'early' endosomes do not.</p> Signup and view all the answers

    What is the primary role of the clathrin-coated vesicles in endomembrane trafficking?

    <p>They are involved in packaging receptor/protein complexes for transport to the endosome.</p> Signup and view all the answers

    What kind of cells would have numerous Golgi stacks due to high secretion levels?

    <p>Salivary gland cells and immune system cells exhibit many Golgi stacks.</p> Signup and view all the answers

    What type of molecules can be tagged to assist in vesicle trafficking aside from proteins?

    <p>Membrane lipids can also carry tags, such as phosphate groups, for proper destination targeting.</p> Signup and view all the answers

    What does the pH of the trans-Golgi compartment (approximately 6.4) facilitate?

    <p>It aids in the binding of lysosomal enzymes to their specific receptors.</p> Signup and view all the answers

    What does the retention tag RXR promote in proteins within the endoplasmic reticulum?

    <p>The RXR sequence promotes the retention of proteins in the endoplasmic reticulum.</p> Signup and view all the answers

    Why is it essential that lysosomes do not digest themselves?

    <p>Lysosomes possess a unique membrane and environment that protects them from their own enzymes.</p> Signup and view all the answers

    Describe the process of endocytosis in the context of endosome formation.

    <p>Endocytosis involves the internalization of extracellular materials into membrane-bound vesicles.</p> Signup and view all the answers

    What is the function of hydrolytic enzymes found within lysosomes?

    <p>They degrade all major classes of biological macromolecules into their monomeric units.</p> Signup and view all the answers

    What role do peroxisomes play in fatty acid oxidation in animal tissues?

    <p>Peroxisomes are responsible for about 25-50% of fatty acid oxidation, especially important for degrading long-chain and branched fatty acids.</p> Signup and view all the answers

    How do peroxisomes enhance cellular safety regarding hydrogen peroxide?

    <p>Peroxisomes contain catalase, which detoxifies hydrogen peroxide by breaking it down within the organelle, preventing exposure to the rest of the cell.</p> Signup and view all the answers

    Describe the main difference between the origin of peroxisomes and lysosomes.

    <p>Peroxisomes are not derived from the endoplasmic reticulum and do not belong to the endomembrane system, unlike lysosomes which are part of it.</p> Signup and view all the answers

    What is the significance of the glycocalyx layer associated with lysosomal membrane proteins?

    <p>The glycocalyx protects lysosomal membrane proteins by preventing their degradation by the digestive enzymes inside lysosomes.</p> Signup and view all the answers

    Explain how lysosomes can contribute to extracellular digestion.

    <p>Lysosomes can release their enzymes through exocytosis, enabling the digestion of substances outside the cell, as seen in sperm interactions with egg membranes.</p> Signup and view all the answers

    What types of cells are especially prominent in containing peroxisomes?

    <p>Peroxisomes are especially prominent in mammalian kidney and liver cells, as well as in photosynthetic cells of plants.</p> Signup and view all the answers

    How do peroxisomes protect cells from oxidative damage?

    <p>Peroxisomes detoxify reactive oxygen species and other toxic substances through oxidative reactions, thereby reducing oxidative damage.</p> Signup and view all the answers

    How do Golgi stacks in salivary gland cells relate to protein secretion?

    <p>Salivary gland cells have many Golgi stacks because they secrete large amounts of digestive enzymes, requiring extensive processing of proteins.</p> Signup and view all the answers

    What role do clathrin-coated vesicles play in endosomal trafficking?

    <p>Clathrin-coated vesicles encase receptor/protein complexes and transport them from the trans-Golgi to endosomes for sorting.</p> Signup and view all the answers

    Why are mannose-6-phosphate tags important for lysosomal proteins?

    <p>Mannose-6-phosphate tags target soluble lysosomal proteins to specific receptors in the Golgi, ensuring their delivery to lysosomes.</p> Signup and view all the answers

    What distinguishes early endosomes from late endosomes?

    <p>Early endosomes are involved in sorting and recycling materials internalized via endocytosis, while late endosomes fuse with lysosomes for degradation.</p> Signup and view all the answers

    What mechanisms prevent lysosomes from digesting themselves?

    <p>Lysosomes are protected by their membrane and contain specific glycoproteins, which inhibit the action of their own digestive enzymes.</p> Signup and view all the answers

    How do retention tags like RXR function in the endoplasmic reticulum?

    <p>Retention tags such as RXR promote the retention of proteins within the endoplasmic reticulum, preventing their premature exit.</p> Signup and view all the answers

    What is the significance of the pH of the trans-Golgi compartment?

    <p>The pH of approximately 6.4 in the trans-Golgi facilitates the binding of soluble lysosomal enzymes to their receptors.</p> Signup and view all the answers

    What happens to proteins after they are modified and packaged into vesicles from the Golgi apparatus?

    <p>Modified proteins are sorted and delivered to their specific destinations, such as organelles or the cell membrane, via transport vesicles.</p> Signup and view all the answers

    Study Notes

    Endomembrane Trafficking

    • Proteins are tagged with specific signals, such as amino acid sequences or carbohydrate side chains, which direct them to transport vesicles for movement within the cell.
    • These tags can also prevent proteins from entering certain vesicles.
    • Membrane lipids can also be tagged with phosphate groups or have variations in their length and saturation to aid in vesicle trafficking.

    Golgi Apparatus

    • Proteins are sorted and packaged into vesicles at the trans face of the Golgi.
    • These vesicles deliver their contents to different parts of the cell or fuse with the plasma membrane for secretion.
    • Cells that secrete large amounts of proteins, like salivary glands, have many Golgi stacks.
    • Plant cells use the Golgi to synthesize polysaccharides for cell wall construction and other cellular functions.
    • Some proteins remain in the ER due to a tripeptide sequence RXR (Arg-X-Arg) that promotes retention.
    • Resident proteins of the Golgi apparatus have specific retention tags.

    Protein Targeting

    • Proteins leaving the Golgi are directed to their specific destinations, such as endosomes, lysosomes, or secretory vesicles.
    • Soluble lysosomal proteins are recognized by mannose-6-phosphate tags on their carbohydrate side chains.
    • These tags bind to specific receptors on the trans-Golgi membrane at a pH of ~6.4, facilitating their transport.
    • Receptor-protein complexes are packaged into clathrin-coated vesicles and transported to endosomes.

    Endosomes

    • These are membrane-bound vesicles formed during endocytosis.
    • They sort and deliver protein and lipid vesicles and their contents to and from the plasma membrane.
    • At endosomes, internalized contents are either recycled to the plasma membrane or sorted for degradation.
    • Early endosomes are located near sites of active endocytosis and sort lipid, protein, and extracellular molecules.
    • Late endosomes fuse directly with lysosomes, forming a hybrid organelle.

    Lysosomes

    • These single-membrane organelles contain digestive enzymes for breaking down all major biological macromolecules.
    • The enzymes degrade materials taken in by endocytosis and digest damaged or unnecessary intracellular structures.
    • Lysosomal enzymes are synthesized on the rough ER.
    • Lysosomes fuse with endosomes, aiding in the digestion of the endosome's contents.
    • They are protected from self-digestion by a glycocalyx layer on their membrane, which prevents the enzymes from reaching the proteins.
    • Lysosomes can release their enzymes through exocytosis, allowing for extracellular digestion.
    • This process plays a role in sperm function and can contribute to inflammatory diseases, like rheumatoid arthritis.

    Peroxisomes

    • These organelles are responsible for oxidative reactions using molecular oxygen.
    • They are bound by a single membrane but are not derived from the ER or part of the endomembrane system.
    • They are found in all eukaryotic cells but are more prevalent in certain tissues, like mammalian liver and kidney cells, algae, and plant cells.
    • Peroxisomes are generally smaller than mitochondria.
    • Peroxisomes contain catalase, an enzyme that breaks down hydrogen peroxide (H2O2), a toxic byproduct of oxidative reactions.
    • They also detoxify other reactive oxygen species and toxic substances.
    • Peroxisomes play a role in fatty acid oxidation, especially long-chain and very long-chain fatty acids.
    • They produce acetyl-CoA, which is then exported to the cytosol for energy production or biosynthesis.

    Endomembrane Trafficking

    • Proteins destined for specific locations contain "tags"
      • Tags can be amino acid sequences, oligosaccharide side chains, hydrophobic domains, or other structural features
      • Tags can also exclude proteins from certain vesicles
    • Membrane lipids can also be tagged for vesicle trafficking
      • Tags can be phosphate groups
      • Length and saturation of membrane lipids also play a role
    • Modified proteins are packaged into vesicles at the Golgi apparatus' trans face
      • Vesicles deliver contents to other organelles or fuse with the plasma membrane for secretion
      • Inhibition of inositol kinases in mammalian cells disrupts vesicle trafficking to the lysosome

    Endosomes & Lysosomes

    • Endosomes are membrane-bound vesicles formed during endocytosis.
    • Endosomes sort and deliver proteins, lipids, and extracellular molecules.
    • Endosomes are involved in recycling back to the plasma membrane or sorting for degradation.
    • Early endosomes are near sites of active endocytosis and sort incoming materials.
    • Late endosomes fuse with lysosomes to form hybrid organelles.

    Lysosomes

    • Lysosomes are membrane-bound organelles containing digestive enzymes capable of breaking down macromolecules like lipids, carbohydrates, nucleic acids, and proteins.
    • Lysosomes degrade extracellular materials taken in by endocytosis and digest damaged or unnecessary intracellular structures and macromolecules.
    • Lysosomal enzymes are synthesized by the rough ER.
    • Lysosomes fuse with endosomes to aid digestion of internalized material.

    Peroxisomes

    • Peroxisomes are single-membrane organelles specialized for oxidative reactions using molecular oxygen.
    • Peroxisomes are not derived from the ER and are not part of the endomembrane system.
    • They are found in all eukaryotic cells, but are particularly prominent in kidney and liver cells of mammals, algae, photosynthetic plant cells, and germinating plant seedlings.
    • Peroxisomes are typically smaller than mitochondria, but size can vary depending on function and tissue.
    • Peroxisomes contain catalase, an enzyme critical for degrading hydrogen peroxide, which is a byproduct of oxidases.
    • The generation and degradation of hydrogen peroxide occur within the peroxisome to protect other cell parts from exposure.
    • Peroxisomes neutralize reactive oxygen species like superoxide anion and hydroxyl radical.
    • Peroxisomes help detoxify harmful substances like methanol, ethanol, formic acid, formaldehyde, nitrites, and phenols.
    • Peroxisomes contribute to fatty acid oxidation, particularly in animal cells for long, very long, and branched chain fatty acids.
    • Peroxisomal β oxidation yields acetyl-CoA, used for biosynthesis or the citric acid cycle.
    • In plants and yeasts, peroxisomes are essential for the complete breakdown of all fatty acids.

    Endomembrane Trafficking

    • Each protein contains a specific "tag" targeting the protein to a transport vesicle.
    • Depending on the protein and its destination, the tag may be a short amino acid sequence, an oligosaccharide side chain, a hydrophobic domain, or some other structural feature.
    • Tags may also be involved in excluding material from certain vesicles.
    • Membrane lipids can also be tagged to help vesicles reach their proper destinations.
      • This tag can be one or more phosphate groups attached to the molecules.
    • The sorting and packaging of modified proteins into vesicles bud from the trans face of the Golgi.
    • Some of these vesicles deliver their contents to other parts of the cell.
    • Others fuse with the plasma membrane, secreting out of the cell.
    • Inhibition of inositol kinases in mammalian cells disrupts vesicle trafficking to the lysosome.
    • Cells that secrete many proteins have many Golgi stacks.
    • In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides.
    • Some proteins retain in the ER contain the tripeptide sequence RXR (Arg-X-Arg), which appears to promote retention in ER.
    • Some resident proteins of the Golgi apparatus also contain retention tags.

    Protein Targeting

    • Proteins that move out of the Golgi apparatus must be targeted to end up in the right place, such as endosomes, lysosomes, secretory vesicles, etc.

    • Soluble lysosomal proteins are targeted by recognition of mannose-6-phosphate in the carbohydrate side chains by specific receptors on the membrane of the trans-Golgi compartment.

      • pH in the trans-Golgi is ~6.4 favoring binding of soluble lysosomal enzymes to these receptors.

    • These receptor/protein complexes are then packaged into clathrin-coated transport vesicles and conveyed to an endosome.

    Endosomes

    • Endosomes are membrane-bound vesicles formed upon endocytosis.
    • Endosomes are typically involved in sorting and delivery of protein and lipid vesicles and their contents to and from the plasma membrane.
    • At endosomes, the internalized contents are either recycled back to the plasma membrane or are sorted for degradation.
    • The ‘early’ endosomes are positioned close to sites of active endocytosis.
    • The “late” endosomes fuse directly and completely with lysosomes to form a hybrid organelle.

    Lysosomes

    • The lysosome is a membrane-bound organelle of the endomembrane system that contains digestive enzymes.

    • They degrade all the major classes of biological macromolecules—lipids, carbohydrates, nucleic acids, and proteins.

    • These hydrolytic enzymes degrade extracellular materials brought into the cell by endocytosis and digest intracellular structures and macromolecules that are damaged or no longer needed.

    • Lysosomal enzymes are synthesized by ribosomes attached to the rough ER.

    • Lysosomes fuse with endosomes, aiding the digestion of materials inside the endosome.

    • Most of the proteins found in the membrane of a lysosome have an unusually large number of carbohydrates attached to them.

      • These carbohydrate groups protect the membrane proteins by preventing the digestive enzymes in the interior of the lysosome from breaking them down.

    • In some cases, lysosomes discharge their enzymes to the outside of the cell by exocytosis, resulting in extracellular digestion.

      • The head of the sperm releases lysosomal enzymes capable of degrading barriers that would otherwise prevent contact between the sperm and egg membranes.

    • Certain inflammatory diseases, such as rheumatoid arthritis, may result from the inadvertent release of lysosomal enzymes by white blood cells in the joints, damaging the joint tissue.

    Peroxisomes

    • Peroxisomes are specialized for carrying out oxidative reactions using molecular oxygen.
    • Peroxisomes are bounded by single membranes.
    • Peroxisomes are NOT derived from the ER and are NOT part of the endomembrane system.
    • They are found in all eukaryotic cells but are especially prominent in mammalian kidney and liver cells, in algae and photosynthetic cells of plants, and in germinating seedlings of plant species.
    • They are somewhat smaller than mitochondria, though there is considerable variation in size, depending on their function and the tissue where they are found.

    Hydrogen Peroxide (H2O2) Metabolism

    • H2O2 is a potentially toxic compound formed by oxidases.
    • The peroxisome contains catalase, an enzyme essential for the degradation of hydrogen peroxide.
      • Detoxification of H2O2.
      • Both catalases and oxidases are confined to peroxisomes.
    • The generation and degradation of H2O2 occur within the same organelle, thereby protecting other parts of the cell from exposure to this harmful compound.
    • Peroxisomes detoxify reactive oxygen species such as superoxide anion and hydroxyl radical, as well as other toxic substances, e.g., methanol, ethanol, formic acid, formaldehyde, nitrites, and phenols.
      • This reduces oxidative damage to cellular components.

    Oxidation of Fatty Acids

    • About 25–50% of fatty acid oxidation in animal tissues occurs in peroxisomes.
      • The remainder is localized in mitochondria.
      • This provides energy for the cell.
    • In animal cells, peroxisomal β oxidation is especially important for degrading long-chain (16–22 carbons), very long chain (24–26 carbons), and branched fatty acids.
    • The primary product of β oxidation, acetyl-CoA, is then exported to the cytosol.
      • It enters biosynthetic pathways or the citric acid cycle.
    • In plants and yeasts, peroxisomes are essential for the complete catabolism of all fatty acids to acetyl-CoA.

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    Description

    This quiz explores the mechanisms of endomembrane trafficking, focusing on protein tagging and the functions of the Golgi apparatus. Learn how proteins and lipids are sorted, packaged, and delivered within the cell, and the implications of these processes in various cell types. Ideal for students studying cell biology and membrane dynamics.

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